Ab initio study of anomeric effect in 2,2-difluoroglycine

The optimized molecular structures of seven conformations of 2,2-difluorogl ycine have been obtained from ab initio calculations. For conformers in whi ch the lone pair of electrons on the nitrogen are antiperiplanar to one of the C-F bonds, that C-F bond is longer than the other C-F bond, which is sy nperiplanar to the lone pair of electrons. Conformers which have these feat ures are the most stable conformers of those examined. This observation is explained in terms of an anomeric effect of the 1p(N) --> sigma*(C-F). At t he MP2/6-31G* level of calculation, conformers IV and V are 21.5 and 18.7 k J/mol, respectively, more stable than the least stable conformer, VI, which does not exhibit an anomeric effect. Conformer VII was found to be excepti onally stable, in addition to an anomeric effect, this conformer also exhib its features of a F ... H-O hydrogen bond. (C) 2000 Elsevier Science B.V. A ll rights reserved.